As people's living standard is improved, a rate of incidence of diabetes is gradually increasing. According to statistics, at present there are approximately 40 million diabetes patients in China. Studies show that injection of insulin at an early stage assists in protecting the function of a patient's islet β cells and preventing occurrence of complications. Therefore, the most important measure for treating diabetes is to inject insulin. The conventional insulin syringes are usually confronted with the problem of reusability and a high scrappage during manufacture. Currently, there are some self-lock insulin syringes available in the market in which during injection, a force for pushing the piston is not consistent, and when a liquid medicament is right injected up, the pushing force must be increased a little so that an instant speed of the liquid medicament entering the patient's body is quickened, thereby causing pains to the patient. There are also some self-destroying insulin syringes in which an aperture is formed through the rubber piston after use. Also, some syringes, which are provided with safety devices to avoid a needle from pricking a patient's body due to exposure during operation, by no means achieves an object of self-destruction. Since a barrel of an insulin syringe has a smaller inner diameter, it is not easy to place a reasonable self-destroying mechanism in the barrel with such smaller inner diameter. Besides, the needles of the conventional insulin syringes are mostly fixed together with the barrel so that needles of different specifications cannot be changed as desired to meet different needs of the broad mass of patients. Up to now, in the market there is not yet developed a self-destroying insulin syringe in which needles of different specifications can be changed as desired and the needles can be retracted into the barrel upon completion of injection.